As automation of control has been developed, a demand has increased for safety and reliability of electronic control devices. In order to secure the safety of the electronic control devices, it is necessary to promptly detect an abnormality and stop operation when the abnormality occurs. A relay is often used in order to separate the device when the abnormality occurs. For example, a relay sometimes interrupts a main power source when an abnormality occurs. In a motor driving device (in particular, an electric power steering system), a relay sometimes interrupts driving current output (phase output) with respect to a motor when an abnormality occurs.
Meanwhile, replacing the above relay with a semiconductor element has been developing high reliability, a long lifetime, miniaturization, high speed protective operation.
PTL 1 below discloses a technique in which, in an electric power steering system, a metal-oxide-semiconductor field-effect transistor (MOSFET) is inserted into phase output of a power converter and the MOSFET is turned off when an abnormality occurs so as to separate a motor and an inverter.
PTL 2 below discloses, in FIG. 4, a technique in which two MOSFETs are disposed between a power source and a power converter so that directions of diodes are opposite to each other (more specifically, a direction in which current passes through the one MOSFET and a direction in which current passes through the other MOSFET are outward each other). In a case where a MOSFET is used as a relay, even when the MOSFET has been turned off, current flows due to a parasitic diode of the MOSFET. In PTL 2, the two MOSFET are connected in series so as to arrange a direction of a parasitic diode in the one MOSFET and a direction of a parasitic diode in the other MOSFET opposite to each other.
PTL 3 below (refer to FIG. 2) discloses a technique in which a relay interrupts a charge pump power source so as to securely interrupt a MOSFET.
Not only securely stopping operation for safety is required when a failure occurs, but also continuing the operation is required while securing the safety. Electric power systems have been applied to large-sized heavier vehicles as performance of the electric power steering systems has been improved. When operation is stopped upon occurrence of a failure, large steering effort using man power is required. Therefore, even when the failure occurs, continuing the operation is required while securing the safety.
PTL 4 below discloses a method in which when an abnormality occurs in one-phase in a three-phase brushless motor to which star connection is performed in an electric power steering system, magnitude and a direction of a driving current in each of the remaining two phases are controlled so as to rotate a magnetic field vector generated inside the motor. Furthermore, PTL 5 below discloses a packaging method in which totally four bridges including a neutral point are modularized.